I am very sorry for the situation your son is in. It is tough on parents too.
Is it possible for your son to get involved with extracurricular activities or clubs? In those types of situations, the kids often have an obvious similar interest to build upon. Scouts or 4H, science clubs, sports, etc. These can all be places where your son can try to build friendships and social skills. Sometimes, club friendships will carry over to school, but even if they do not, he might find a place he feels at home.
One place where we often saw like minded children was at the library. If you see a possibility where your son seems to be having fun talking to another child, maybe go find the parent and see if the kids can meet up again at the same place.
As far as the school bullying situations, you may need to step in and help him. Model how to respectfully bring these to the attention to the teachers. I agree that sometimes, teachers do blame the excluded child, but you might get lucky enough to find a teacher who sees through that and can help.
